1. Field of the Invention
This invention relates to improvements in plural radio altimeter systems, and, more particularly, to an improved method and apparatus for reducing interference between a plurality of radio altimeters.
2. Description of the Prior Art
Multiple navigation sensors are becoming of widespread use, especially in automatic flight control systems. Typically, flight control systems rely upon dual or triple sensors, the outputs of which in some cases are voted upon to produce the most likely accurate signal upon which the flight control system can rely for operation of the aircraft.
Continuous wave frequency modulated (CWFM) radio altimeters are also now of widespread use. Their operation is briefly as follows. A transmitter signal of approximately 4300 MHz is modulated with a modulation frequency which changes in accordance with a sawtooth function to sweep the transmitter signal over about 100 MHz at a rate of about 100 Hz. The transmitted signal is directed to the underlying terrain, from which reflections are received and mixed with a currently transmitted signal. The difference in the transmitted and received frequencies is relatable to the two-way travel time or propagation delay, and can be decoded to indicate altitude, all as is known in the art.
In plural or redundant systems, two or more such radio altimeters are used, often with their transmitter and receiver antennas physically located side-by-side. Although the placement of the antennae of plural altimeter systems plays a large part in avoiding mutual interference between them, the modulation phase differential of the redundant altimeters are usually carefully controlled, for if such control is not properly achieved, each receiver may detect the signals of the other transmitter and produce an erroneous reading. Such erroneous reading may furthermore be produced simultaneously by both altimeters, each interferring with the other and appearing correct, since the erroneous indication redundantly appears in the system. Typically, however, when one of the transmitter modulation frequencies of the radio altimeters moves into phase alignment with the other, it coincides entirely with the frequencies of the other transmitter. This results in both altimeters rapidly showing increased altitude indications as the frequencies move into coincidence, then a rapid return to the original altitude indication after the frequencies cross. This phenomenon is generally violently exhibited, and its display is undesirably disconcerting to flight crews.
To solve this problem, transmitter interconnections have been used to coordinate the modulation frequency sweeps and force a fixed phase offset of the respective altimeters' transmitters. However, if one of the radio altimeters should fail, such interconnections may cause the others in the system also to fail, or render the entire system unusable.